net/http/httputil: fix race in DumpRequestOut

Fixes #2715

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/5614043

src/pkg/net/http/httputil/dump.go

@@ -5,8 +5,8 @@
 package httputil
 
 import (
+	"bufio"
 	"bytes"
-	"errors"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -47,40 +47,59 @@ func (c *dumpConn) SetWriteDeadline(t time.Time) error { return nil }
 // DumpRequestOut is like DumpRequest but includes
 // headers that the standard http.Transport adds,
 // such as User-Agent.
-func DumpRequestOut(req *http.Request, body bool) (dump []byte, err error) {
+func DumpRequestOut(req *http.Request, body bool) ([]byte, error) {
 	save := req.Body
 	if !body || req.Body == nil {
 		req.Body = nil
 	} else {
+		var err error
 		save, req.Body, err = drainBody(req.Body)
 		if err != nil {
-			return
+			return nil, err
 		}
 	}
 
-	var b bytes.Buffer
-	dialed := false
+	// Use the actual Transport code to record what we would send
+	// on the wire, but not using TCP.  Use a Transport with a
+	// customer dialer that returns a fake net.Conn that waits
+	// for the full input (and recording it), and then responds
+	// with a dummy response.
+	var buf bytes.Buffer // records the output
+	pr, pw := io.Pipe()
+	dr := &delegateReader{c: make(chan io.Reader)}
+	// Wait for the request before replying with a dummy response:
+	go func() {
+		http.ReadRequest(bufio.NewReader(pr))
+		dr.c <- strings.NewReader("HTTP/1.1 204 No Content\r\n\r\n")
+	}()
+
 	t := &http.Transport{
-		Dial: func(net, addr string) (c net.Conn, err error) {
-			if dialed {
-				return nil, errors.New("unexpected second dial")
-			}
-			c = &dumpConn{
-				Writer: &b,
-				Reader: strings.NewReader("HTTP/1.1 500 Fake Error\r\n\r\n"),
-			}
-			return
+		Dial: func(net, addr string) (net.Conn, error) {
+			return &dumpConn{io.MultiWriter(pw, &buf), dr}, nil
 		},
 	}
 
-	_, err = t.RoundTrip(req)
+	_, err := t.RoundTrip(req)
 
 	req.Body = save
 	if err != nil {
-		return
+		return nil, err
 	}
-	dump = b.Bytes()
-	return
+	return buf.Bytes(), nil
+}
+
+// delegateReader is a reader that delegates to another reader,
+// once it arrives on a channel.
+type delegateReader struct {
+	c chan io.Reader
+	r io.Reader // nil until received from c
+}
+
+func (r *delegateReader) Read(p []byte) (int, error) {
+	if r.r == nil {
+		r.r = <-r.c
+	}
+	return r.r.Read(p)
 }
 
 // Return value if nonempty, def otherwise.